The invention relates to woven permeable fabric which supports paper stock during the manufacture of paper on a papermaking machine. In particular, the invention is directed to a multilayer fabric having increased structural stability in a machine direction in which the fabric travels on the papermaking machine while still affording a high degree of permeability which facilitates drying of the paper. The fabric of the invention has application as a support fabric for directly supporting a paper web on a papermaking machine. The fabric has further application as a carrier fabric for carrying a layer of material which contacts the paper instead of the paper contacting the fabric directly. A carrier fabric is typically utilized in the manufacture of embossed paper products as a base fabric. In such an application, a layer of material is embedded in or carried on the base fabric which is embossed to imprint a desired pattern on the paper sheet contacted by the embossed layer. The load in the machine direction is carried mainly by the base fabric and not the embossed layer. For drying purposes, the carrier fabric must have a high degree of openness and air permeability so that sufficient air is delivered through the base fabric and the embossed layer, which is also permeable for drying. Carrier fabric must have sufficient load bearing capability for bearing the loads in the machine direction which are most severe.
Heretofore, single layer fabrics have been utilized as carrier and support fabrics which have one warp system and one weft system. In order for a single layer of fabric to have an open area above thirty percent the machine direction yarns become spread apart to such an extent that fabric stability in the machine direction becomes too low. In order to achieve desired projected open areas above thirty percent a single layer fabric must be made of thin warp and weft yarns (e.g. 0.10 to 0.20 mm diameter). The single layer fabrics have utilized low warp and weft counts per centimeter, for example, 20 ends or picks per centimeter. Under these conditions, the single layer fabric tends to stretch unacceptably while traveling in the machine direction. If additional machine direction yarns are utilized in order to strengthen the fabric, the open area of the fabric is reduced resulting in the permeability of the fabric being below desired levels.
A single layer fabric is disclosed in U.S. Pat. No. 4,281,688 having a plurality of dominating floats on opposing faces of the fabric. Every alternating weft has a long knuckle to one face, and every other weft has a long knuckle to the opposite face. The projected open area of the fabric is limited.
U.S. Pat. No. 4,314,589 discloses a double layer fabric having two weft layers and a single warp layer. The warps lie next to each other almost without any spacing between adjacent warps providing little or no projected open area. U.S. Pat. No. 4,359,069 discloses a double layer fabric having a single warp yarn system extending in the machine direction and a double layer weft yarn system in the cross-machine direction. The yarns of the single layer warp system are spaced apart from one another with a yarn density of 0.50 to 0.65. This warp density in the machine direction cannot be lowered, as otherwise the fabric stability would drop too much. This provides a projected open area of only 13 to 25 percent of the total fabric area. The warp yarns in the machine direction have to bear the load when the fabric runs on the papermaking machine. U.S. Pat. No. 4,359,069 teaches recessing the single layer warp system which extends in the machine direction between the two layers of the weft yarn so the warp yarns are removed from wear, it is thought that this will enable the warp yarns to better withstand the longitudinal stresses and provide a longer fabric life. U.S. Pat. No. 4,344,465 discloses a double layer forming fabric having two function sides. However, there is only one layer of load bearing machine direction yarns. There are machine direction yarns on the paper support side of the fabric which do not bear loads.
International Publication No. (PCT) WO 80/01086, U.S. Pat. No. 4,356,225, and European Patent Application No. EP 0 123 431 A2, describe multilayer wet felt designs. The technology for weaving multilayered fabrics for felt bases was begun primarily to increase void volume under pressure. These press felt base fabrics are preferably woven endless. Due to the quite different objectives in designing these fabrics none of these described designs show a structurally stable weave pattern and a projected open area in the range of thirty percent or more as in the case of the present invention.
European Patent Application No. EP 0 135 231 A1 discloses a single layer flat carrier fabric used as a carrier of an embossed layer which imprints paper.
Thus, it can be seen that the prior single layer and multilayer fabrics are limited in their capacity to provide both high degrees of projected open area and structural stability in the machine direction.
Accordingly, an important object of the present invention is to provide a method and fabric with improved fabric stability in the machine direction while maintaining a projected open fabric area which facilitates use of the fabric as a support or carrier fabric on papermaking machines.
Still another important object of the present invention is to provide a woven multilayered papermaking fabric having an increased number of load bearing longitudinal yarns, i.e. yarns extending in a machine direction while maintaining a sufficient distance between adjacent longitudinal yarns to allow for a projected open area of at least thirty percent of the total fabric area.
Still another important object of the present invention is to provide a highly permeable woven fabric for use on paper machines and the like and method therefor wherein the load bearing machine direction yarns are doubled in their density without a decrease in the projected open area of the fabric.
Yet another important object of the present invention is to provide a woven multilayered papermaking fabric having a first layer and a second layer of longitudinal yarns, which are interwoven with a single transverse yarn system which maintains the longitudinal yarns of the first and second layers in stacked pairs which may be spaced apart sufficiently to provide a desired open area in the fabric.